The invention relates to a piezoelectric glass ceramic which is translucent at least in the visible light range or in the infrared range.
The invention relates to a piezoelectric glass ceramic which is translucent at least in the visible light range or in the infrared range.
Piezoelectric materials are intensively used worldwide, due to their unique material's performance. Piezoelectric glass ceramic materials provide an interesting and promising alternative to conventional piezoelectric materials such as PZT ceramics. Since PZT apart from zirconium and titanium contains lead, piezoelectric glass ceramics have a potential to replace PZT as an alternative lead-free material.
The term piezoelectricity refers to the ability of a material to produce measurable electrical charge as a result of an applied stress (or the converse—an applied electrical charge (field) results in a change of shape of the material). For such functionality to exist, however, one main constraint must be met: the material to give rise to a piezoelectric response must lack a center of symmetry (COS). This immediately rules out all glasses and many different types of crystals, but not all. In particular, crystals that belong to those point groups that lack a COS (“acentric”) are piezo-active. Moreover, other classes of materials are also piezo-active, including various organic materials. This reliance on a lack of symmetry, thus, requires that a glass contains acentric crystals in order for the composite material, a glass ceramic, to be potentially piezo-active. However, this is only a necessary condition for functionality, not a sufficient one. For the material to be functional, the composite material itself must lack a COS. For example, a single crystal of NaNbO3 is acentric and thus, by definition, lacks a COS. However, a glass ceramic (or ceramic for that matter) containing randomly-oriented crystals of NaNbO3 has a COS. To break this symmetry requires either (1) crystal alignment, typically during the ceramization process of a glass ceramic; or (2) internal domain alignment via the application of an electric field (so-called “poling”) which is only available for ferroelectric materials. In so doing, the material would now be acentric and should thus exhibit measurable piezoelectricity. “Ferroelectric” materials are those that have internal domains whose orientation can be switched (and aligned) during the application of an electric field. PZT is one of the common conventional material of the ferroelectric type. However, ferroelectricity is not a requirement for piezo-activity, and thus there are many types of crystals (e.g. quartz, lithium disilicate, etc.) that are non-ferroelectric but are piezo-active. For these types of materials, crystal alignment is necessary to achieve a functional material.
Halliyal et al., “Glass ceramics for piezoelectric and pyroelectric devices”, in Glass and Glass-ceramics, edited by M. H. Lewis, pp. 273-315, Chapman and Hall, London, 1989, investigated a variety of glass ceramics showing piezoelectric or pyroelectric behavior. In particular, they investigated a glass-ceramic material prepared from lithium borosilicate precursor glasses (Li2O—B2O3—SiO2). Halliyal et al. used a crystallization in a temperature gradient which was generated by positioning polished glass samples on a microscope hot stage. Thereby piezoelectric samples could be prepared from non-ferroelectric piezo-active materials by effecting a preferred direction of orientation of the precipitated crystallites. Halliyal et al. reported a piezoelectric charge constant d33 (parallel to the crystallization direction) in the range of 5 pCN−1. The diameter of the crystallites generated by the hot stage method was generally in the range of 1 to 3 micrometers. The samples produced by the hot stage method all became opaque and milky-white after crystallization (Halliyal 1984, The Study of the piezoelectric and pyroelectric properties of polar glass ceramics, Ph.D. thesis, The Pennsylvania State University, 1984).
However, for various applications of piezoelectric glass ceramics it would be highly desirable to provide materials which are translucent, at least to a certain extent within the visible (VIS) range or within the infrared (IR) range.
By H. Jain, “Translucent ferroelectric glass ceramics”, Ferroelectrics vol. 306, 2004, 111-127, translucent ferroelectric glass ceramics were investigated. Jain reported that barium titanate and various niobate glass ceramics are translucent only, if the crystallite size is smaller than 0.2 micrometer. However, such samples did not show typical ferroelectric hysteresis or dielectric constant peak at the ferroelectric transition temperature. According to Jain in general, devitrification of glass increases optical loss from enhanced scattering. Jain reported that pyroelectric measurements on a devitrified CdO doped NaNbO3—SiO2 translucent composition where believed to indicate that crystalline aggregates of less than 100 nm were ferroelectric.
However, this ferroelectricity was not demonstrated for a glass ceramic in general, but only for some devitrified samples. Also translucency of the respective samples was not confirmed.